As technology progresses and the world becomes more populated, more waste is produced each day in homes, offices and industrial plants. In the past, waste has been dumped into landfills located near most municipalities or into the oceans, with little regard for the physical space consumed or the potential damage done to the environment. More recently land space allocation and environmental damage have become substantial public concerns.
The prior art, as does the present invention, recognizes that if waste is transported to a central location, pyrolysis and vitrification can be accomplished, utilizing plasma arc heating technology, in an efficient and safe manner and useful gaseous and vitrified products produced so as to avoid placing the waste residue into a landfill. The invention disclosed presents a versatile method for the handling of mixed waste which improves on earlier systems and which can be adapted to the requirements of the particular quantity of waste to be processed.
As described in U.S. Pat. No. 5,280,757 and other prior art noted below, plasma arc heated processes are receiving considerable attention for waste treatment over fuel combustion heated processes because of several distinct advantages of plasma heat which is well suited for the pyrolysis and vitrification of waste materials. A plasma arc torch operates by supporting a high voltage electric arc on a flow of plasma (ionized) gas to generate an extremely hot "flame". The quantity of plasma gas flowing through the plasma torch is significantly less than the quantity of gas required to release the equivalent heat energy by the combustion of hydrocarbon fuels. A further difference and advantage of a plasma torch heat source over a combustion heat source is that the plasma torch can be used to produce useful by-product gases of higher caloric content referred to here as the degassing process. In addition, by virtue of the fact that a plasma arc torch uses only a small quantity of gas to support the arc and generate the heat, combustion is unlikely to occur spontaneously in the materials which are being heated. A major advantage of the plasma torch is that it is capable of unusually high rates of heat transfer, adding to its inherent efficiency. Also, the temperature of 4,000.degree.-7,000.degree. C. generated by a plasma torch is much hotter than that generated by a combustion source and is hot enough to melt any known material simultaneously with the pyrolysis degassing process.
An apparatus and process utilizing plasma arc heating for processing household and industrial waste in a plasma heated reactor is disclosed in U.S. Pat. No. 3,779,182 to the present inventor. The '182 patent is also noted for teaching the introduction of oxygen or air to the reactor. The teachings of the '182 patent are incorporated herein by reference.
The term "reactor" as used herein refers to the process containment vessel, or furnace, into which refuse, e.g. municipal solid waste, is placed and heat is added for the purpose of promoting the simultaneous pyrolysis of organics and vitrification of inorganics of the mixed wastes.
U.S. Pat. No. 5,143,000 to the present inventor describes a plasma arc heated furnace for the treatment of solid waste. Of interest to the present invention is the fact that the '000 patent teaches loading of non-compacted refuse through the top of a reactor. U.S. Pat. No. 5,280,757 to Carter et al. teaches a process for treating solid waste which includes feeding, compressing and forcing a stream of solid waste into the bottom of a reactor vessel heated with a plasma torch. The '757 patent also refers to injecting steam into the reactor. It has not been known, however, to provide a process in which a bulk supply of municipal solid waste is compacted into a substantially air-free elongated stream of substantially uniform cross section which is positioned above the top of the reactor and is discharged in increments as compressed blocks of waste into the top of the reactor. The present invention thus seeks to provide such a process.
Entrapped air, if permitted to enter the reactor with the solid waste, will allow combustion in an uncontrolled process and cause the resultant gases to be both different in nature and non-useful as compared to those resulting from pyrolysis of the organic waste materials alone in a substantially air-free environment. Through pyrolysis of organic waste, the by-product gases are principally components of valuable fuels such as hydrogen and carbon monoxide. The inclusion of a large quantity of air will add a significant quantity of nitrogen that will dilute the energy content of the gas. The present invention recognizes that the admission of a limited and controlled amount of air can be used to advantage, in conjunction with dropping increments of compacted waste of known size into the top of the reactor.
It is therefore an overall object of this invention to provide an improved process for plasma pyrolysis and vitrification, which reduces the volume of input mixed waste materials, and results in by-product gases which have high energy content that can be used.
It is a further object of this invention to provide an improved waste feeding method and process for use with the pyrolysis and vitrification of mixed waste.
It is an additional object of this invention to provide a process which pyrolyzes the organics of mixed waste including the wastes' moisture content and vitrifies the inorganics of mixed waste efficiently based on using an improved waste feeding system in conjunction with introducing controlled amounts of air and recycling the steam derived from the moisture content of the mixed wastes.
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.